Most of the organic fluorescent compounds show good luminescnt properties in dilute solutions. However, when they are in aggregation state, especially in crystalline state, the fluorescence declines or even quenches. This is because molecule interaction increases with the increase of concentration of fluorescent substance, resulting in decrease of fluorescence intensity because of easy formation of excimers between same molecules or exciplexes between different molecules (Nature, 1999, 397, 121-128; Science, 1994, 265, 765-768).
Especially for organic fluorescent molecules with rigid big planar structures, they show strong fluorescence in solutions but very weak fluorescence in aggregates. This phenomenon is known as the aggregation-caused quenching (ACQ) of fluorescence. ACQ restricts the use of fluorescent substances in preparing organic light-emitting diodes (OLED), chemical and biological sensors and in other fields. For example, when manufacturing OLED, which has advantages of low driving voltage, light weight, all-solid self-luminous property, extremely thin thickness, strong brightness, high efficiency, quick responding speed, wide viewing angle, potentiality to make flexible bendable display panel and suitability for large area display, it requires the organic luminescent material to be made into thin solid films or other aggregation forms, which inevitably lead to ACQ and affect the luminous efficiency of the organic fluorescent substance and the service life of the OLED.
As for the use in fluorescent sensors or probes, the concentration of the fluorescent substance used for fluorescent sensors or probes have to be maintained in a very low level because of ACQ, which reduces the accuracy of the method when it is used for quantitative analysis. In addition, the aggregation of the fluorescent substances in active sites of biological macromolecules also significantly affects the accuracy of the detecting method.
Therefore, ACQ has always been a problem that is difficult to overcome but must be resolved in the use of organic fluorescent substance. For a long time, various chemical and physical methods and processing technologies have been used to try to reduce the ACQ of fluorescent substances. However, very limited results were achieved.
In the process of developing various new luminescent materials with more excellent performance, TANG Benzhong and his research group in 2001 found compound silole I (see compound I in FIG. 1) that is not luminous in solutions but shows very strong luminescent property in solid state (Chem. Commun. 2001, 18, 1740-1741; J. Mater. Chem., 2001, 11, 2974-2978). They named this phenomenon as aggregation-induced emission (ATE).
The discovery of AIE phenomenon provides a fundamental solution for the short life of OLED and low detection accuracy of fluorescent sensors or probes caused by ACQ. Therefore, organic fluorescent substances with AIE properties has quickly became the object of studies in fields such as synthetic organic chemistry, luminescent material chemistry, biochemistry, analytical chemistry, etc. In the recent ten years, organic fluorescent substances with AIE properties have showed unique advantages in fields such as OLEDs (Patent CN101659865A, 2010; J. Phys. Chem. C. 2008, 112(10), 3975-3981; J. Am. Chem. Soc. 2002, 124 (22), 6469-6479; J. Am. Chem. Soc. 2005, 127(25), 9071-9078; J. Am. Chem. Soc. 2006, 128(31), 10163-10170), fluorescence sensors (Patent US20080220407A1, 2008; Patent US20100009362A1, 2010; Patent JP2010112777A, 2010; J. Am. Chem. Soc. 2010, 132(40), 13951-13953; Macromolecules 2009, 42(5), 1421-1424; Chem. Commun. 2009, (33), 4974-4976), fluorescent probes (Org. Biom. Chem. 2011, 9(7), 2219-2226; Chem. Commun. 2010, 46(23), 4067-4069; Chem-Eur. J. 2010, 16(28), 8433-8438; Chem. Commun. 2009, (33), 4974-4976; Chem. Commun. 2006, (35), 3705-3707) and biology activity examinations (Org. Lett. 2009, 11(17), 4014-4017; Org. Lett. 2010, 12(10), 2274-2277) (Curr. Org. Chem. 2010, 14(18), 2109-2132; J. Mater. Chem. 2010, 20(10), 1858-1867; Chem. Commun. 2009, (29), 4332-4353; J. Inorg. Organomet. P. 2009, 19(3), 249-285).
Although organic fluorescent substances with AIE properties have broad application prospects in various fields, there are very few types of such compounds. TANG Benzhong and his research group deeply studied the factors that affect the AIE property and the reason why it happens, and proposed the mechanism for generating A1E phenomenon. They think that AIE molecules are not luminous in solutions because they can decay non-radiatively by rotation or vibration of benzene, and becomes luminous in solids because they decay in a radiative form owing to the restriction of the rotation or vibration in aggregates (Angew. Chem. Int. Ed. 2009, 48, 7608-7611; J. Am. Chem. Soc, 2005, 127, 6335-6346; Chem. Mater., 2003, 15, 1535-1546; J. Phys. Chem. C, 2007, 111, 2287-2294).
The known AIE molecular structures have the following two characteristics: (1) the molecule contains at least two or more rotatable and conjugated rigid aromatic rings; (2) the steric effect generated by these aromatic rings makes the whole molecule become a twisted non-planar spatial configuration, instead of the rigid planar spatial molecular configuration of normal organic fluorescent compounds. The results achieved by TANG Benzhong and his research group act as a guide to the design and synthesis of AIE molecules. Many compounds with aggregation induced emission enhancement (AIEE) have been developed in recent years. These compounds have luminescence in solution and their luminescence increases (generally less than 100 times) in aggregation state (J. Mater. Chem. 2011, 21(11), 3760-3767; Chem-Eur. J. 2011, 17(9), 2647-2655; Chem. Commun. 2010, 46(47), 9013-9015; J. Phys. Chem. C, 2010, 114 (43), 18702-18711). But there are very few compounds with AIE property, i.e., compounds that have almost no luminescence in solution but become luminous in aggregation state. There are only a few AIE compounds whose luminous intensity in aggregation state is stronger than their luminous intensity in solution by two orders of magnitude, such as compound I and compound III in FIG. 1.
Based on whether the aromatic rings are connected by single bonds to the double bonds inside the rings or outside the rings, the AIE compounds can be divided into cyclic compounds and chain compounds. Since the cyclic AIE compounds are more difficult to be synthesized than chain AIE compounds, most of the reported AIE compounds are chain compounds. Cyclic AIE compounds are mainly multi-phenyl silicone heterocyclic compounds Silole (such as compound I in FIG. 1, whose fluorescence quantum yield in aggregation state is 330 times more than the fluorescence quantum yield in solution) discovered by TANG Benzhong and his research group (Chem. Commun. 2001, (18), 1740-1741). The replacement of Si atom in Silole by Ge and Sn atoms also provides certain AIE property. But the AIE effect is very weak and the fluorescence quantum yield in aggregation state is only increased by several times (Inorg. Chem. 2005, 44(6), 2003-2011).
Most of the chain AIE compounds can be represented by multi-phenyl vinyl compounds, such as compound II (J. Org. Chem. 2005, 70(7), 2778-2792) and III (J. Am. Chem. Soc. 2002 124(48), 14410-14415) in FIG. 1. There are also some specially structured AIE compounds, such as compounds IV and V, in which two or more benzene rings are connected directly by single bonds and form a conjugated system (the fluorescence wavelength of this kind of compounds is in the non-visible region of the ultraviolet region and the fluorescence quantum yield in aggregation state and in solution are not measured, Chem. Commun., 2007, 70-72), and compound VI with C═N double bonds in the conjugated system (J. Org. Chem. 2009, 74, 2163-2166). Because of the wide use of AIE compounds, it is necessary to develop more AIE fluorescent compound with good luminous efficiency.